I really have no idea on the angle they're gonna go with who's next, but seeing how some big name characters (Mario, Megaman, and Luigi) are already going down, they either will continue this trend or have a hero beat a villain.
Though being one who greatly wants Phosphora, if this was to happen, I can see two main ways going at this; either Pit and Palu fighting together against other characters, only to get ambushed by Phosphora (and Palu being defeated) or a revisit of Pit vs Link except with Pit having the upper hand this time, but Phosphora does the finishing blow to Link...
Whatever path is chosen (I prefer the one where Link goes down though an FE character can take his place), they could reference Pit and Phosphora having a rematch at the end after we see her in action in reference to Uprising, where Pit technically defeated a "weakened" Phosphora.
Sigh, but this is probably just me dreaming of wanting Phosphora as I think she has a chance (not being shown as an AT yet, right?).
Now should Medusa be the choice, She may either take out Palu or go after Pit first as a means of revenge.
But as for potential characters going out next, It's either Link, an FE veteran, or a villain (likely Bowser), but I'm still hoping it's one of the first two... That's not to say that it can't be Bowser or Charizard being "hunted" by a Monster Hunter since that Rathalos showed up... I feel it's more likely if a direct is around late August or early September.